The present invention relates to a crosslinkable acrylic rubber and a method for producing the same. More particularly, the invention relates to a crosslinkable acrylic rubber which can provide a crosslinked product having excellent alkali resistance and oil resistance, and a method for producing the same.
It is known that acrylic rubbers are superior in heat resistance and oil resistance to general rubbers. Sulfur vulcanization is not applicable to the acrylic rubbers because they have no unsaturated double bond. Therefore, in order to make crosslinkable, acrylic rubbers have been produced by copolymerizing alkyl acrylates with comonomers to impart a crosslinkability. For example, acrylic acid esters are copolymerized with halogen-containing monomers such as 2-chloroethyl vinyl ether and vinyl chloroacetate, or epoxy monomers such as allyl glycidyl ether, glycidyl acrylate and glycidyl methacrylate so that the obtained acrylic rubbers can be crosslinked with a crosslinking agent or curing agent, e.g. a fatty acid soap or an amine compound such as ethyltetramine or tetraethylpentamine, as disclosed in Japanese Patent Publication Kokoku No. 57843/1986.
However, these known acrylic rubbers have some defects. The acrylic rubbers prepared by copolymerization with the halogen-containing monomers have the defects that the water resistance is poor due to a reactivity of halogen with water, and that they cause prevulcanization of mixed compound because the monomer units introduced for crosslinking react with a crosslinking agent too fast. The acrylic rubbers prepared by copolymerization with the epoxy-containing monomers have no particularly serious defects, but still have problems that they are inferior in heat resistance to the halogen-containing acrylic rubbers and also are insufficient in water resistance and oil resistance and, therefore, they are not satisfactorily applied to uses which require particularly superior properties.
In recent years, the uses of acrylic rubbers are increasing, and it has been demanded to have an alkali resistance. However, the halogen-containing acrylic rubbers are poor in alkali resistance because a halogen has a reactivity with an alkali. Also, the epoxy-containing acrylic rubbers are insufficient in alkali resistance and cannot meet the demand in this respect, too, in addition to poor heat and oil resistances as mentioned above.
An acrylic rubber prepared by copolymerizing methyl acrylate as a main component with ethylene and a monomer having carboxyl group, is lately put on the market as acrylic rubber having an alkali resistance. This acrylic rubber has a good alkali resistance. However, it is poor in oil resistance, especially in grease resistance, because ethylene is used as a comonomer. Therefore, this acrylic rubber is not suitable for uses which require a grease resistance. Because a monomer having a free carboxyl group is introduced as monomer units to impart a crosslinkability, this acrylic rubber has also the problem that the crosslinking reaction proceeds so fast that the prevulcanization of a mixed compound is easy to occur during roll processing or storing of the mixed compound, thus it is not satisfied in processability.
As mentioned above, conventional acrylic rubbers have any defect in properties such that they are poor in water resistance and alkali resistance, or are unsatisfactory in oil resistance, or they are easy to cause prevulcanization of a mixed compound during roll processing or storing because of a high crosslinking reactivity. No acrylic rubber having both a good alkali resistance and a good oil resistance has been provided.
Accordingly, it is an object of the present invention to provide a crosslinkable acrylic rubber capable of providing a cured rubber having excellent alkali resistance and oil resistance in addition to other properties such as processability, water resistance and heat resistance.
A further object of the present invention is to provide a method for producing a crosslinkable acrylic rubber which has a good processability and can provide a cured rubber having good properties such as alkali resistance, oil resistance, water resistance, heat resistance, weather resistance and ozone resistance.
These and other objects of the present invention will become apparent from the description hereinafter.